1. Field of the Invention
This invention relates to an antiskid control system for adequately controlling the rotational behavior of vehicle wheels during brake application.
2. Description of the Prior Art
Heretofore, antiskid systems of varied kinds have been proposed and arranged in different manners to reduce hydraulic braking pressure for avoiding an inadequate wheel braking state incurring a locked state of vehicle wheels and then to increase the reduced braking pressure to prevent the braking distance from unnecessarily extending. The known antiskid control methods include, for example, a method in which: An electromagnetic valve device which is operable at a relatively high frequency is arranged in a braking pressure increasing system in such a way as to permit selection of a braking pressure increasing rate out of various pressure increasing rates; a pulse train generator, such as a multivibrator or the like, is included in a driving control system provided for the electromagnetic valve device; and the timing at which the electromagnetic valve device is driven by a pulse train signal from the multivibrator is determined on the basis of a signal representing the acceleration of vehicle wheel rotating velocity. This method was disclosed, for example, in British Pat. No. 1,305,430.
In the method of providing an electromagnetic valve device in the hydraulic braking pressure increasing system and operating it by introducing a given pulse train signal presents problem in terms of the durability of the expensive electromagnetic valve device because of a frequent repetition of collision and parting taking place within a very short period of time between a valve seat and a valve body.
FIG. 7 of the accompanying drawings shows by way of example the arrangement of an antiskid control system employing the above-stated conventional hydraulic braking pressure increasing method. The illustration includes a gate valve 1; an electromagnetic valve device for increasing or holding hydraulic braking pressure (hereinafter referred to as a hold valve) 2; an electromagnetic valve device for reducing hydraulic braking pressure (hereinafter referred to as a pressure reducing valve) 3; a main transmission line 4 which connects a master cylinder M/C (not shown) to the wheel cylinder W/C of each vehicle wheel brake device via the gate valve 1 and the hold valve 2 for transmitting the hydraulic braking pressure from the master cylinder M/C.
The gate valve 1 includes cylinders 6 and 7 which are of the same diameter and have a differential pressure operated piston 5 inserted therein. During antiskid control, a valve part 8 of the gate valve 1 is closed when the piston 5 is moved toward the left side in the drawing by a difference between the amounts of hydraulic pressure applied to the two sides of this piston. When this valve part 8 is closed, communication in terms of pressure between the side of master cylinder M/C and the downstream side of the gate valve (on the side of the wheel cylinder W/C) is shut off.
The hold valve 2 is of a normally open type and is arranged to be closed by a hydraulic pressure holding signal S.sub.1 produced from a control circuit 9 which is an electronic control device. With the hold valve 2 thus closed, the main transmission line 4 is blocked and there obtains a hydraulic pressure holding state. The pressure reducing valve 3 is of a normally closed type and is arranged to be opened by a pressure reducing signal S.sub.2 produced from the control circuit 9. With the valve 3 opened, the pressure oil within the wheel cylinder W/C is taken up into a pressure accumulator 12 via a reservoir 10 and a pump 11. Between the gate valve 1 and the hold valve 2, there is provided a bypass line 13 for returning the taken up pressure oil. Reference numerals 14 and 15 denote a check valve and a return valve. A wheel rotating velocity detector (or a speed sensor) 16 is mounted on a wheel 17 and is arranged to supply the control circuit 9 with detection information.
With the antiskid control system arranged in this manner, the hydraulic braking pressure is increased by transmitting the pressure oil taken up into the pressure accumulator 12 to the wheel cylinder W/C by performing an opening/closing change-over operation on the hold valve 2. In that instance, in order to make the pressure increasing rate variable, the signal S.sub.1 to be applied from the control circuit 9 to the hold valve 2 must be arranged in the form of a high frequency pulse train signal. Therefore, there arises the above-stated problem with regard to the durability of this hold valve.